In vivo examination of the eye fundus is used not only for ophthalmological purposes but also for diagnosing other disorders that include hypertension, diabetes, and diseases of the cerebral nerves. One technique used for such examinations involves the physician using a device known as the ophthalmoscope to directly observe the eye fundus. In another method that is extensively applied, a special fundus camera is used to record photographs of the fundus on conventional film. The advances made in recent years by electronics have led to the use of optoelectronic transducers such as imaging tubes and the like in place of the photographic film of the conventional fundus camera, whereby eye fundus information is directly obtained in the form of electric signals which can be processed, stored and displayed on a television monitor or the like.
One innovative development in ophthalmology that has attracted attention is that of an electronic ophthalmoscope that utilizes laser scanning techniques. Such a device is known as a scanning laser ophthalmoscope (hereinafter also abbreviated to "SLO"), and is being developed and improved mainly in the U.S., Germany, France and Japan.
With the first SLOs, a laser beam was passed through the center of the pupil and used to scan the eye fundus two-dimensionally, and the light reflected from the fundus through a larger area around the periphery of the pupil was picked up, photoelectrically converted and amplified, whereby with the fundus illuminated at a low brightness it was possible to display a video image of the fundus on a monitor television in real-time with a high S/N ratio (see Reference (1): U.S. Pat. No. 4,213,678 and Applied Optics Vol. 19 (1980) pp 2991 to 2997).
The feasibility of using an active optical element to compensate for the effect of aberration in the optical system of the eye was studied as a way of achieving a major improvement in fundus image resolution, compared to the conventional fundus camera (see Reference (2): DP 3245939, U.S. Pat. No. 4579430, JP-A-59-115024, SPIE Proceedings Vol. 498 (1984) pp 76 to 82).
The adoption of a confocal optical system in the device arrangement was particularly effective for improving picture quality. This eliminated the effect of stray and scattered light and produced a marked improvement in fundus image contrast and resolution (see Reference (3): FP 2555039, JP-A-60-132536, Journal of Optics (Paris) Vol. 15 (1984) pp 425 to 430; Reference (4): U.S. Pat. No. 4764005, JP-A-62-117524, Applied Optics Vol. 26 (1987) pp 1492 to 1499). In such an arrangement, by simultaneously scanning the incident and reflected light beams (double-scanning) and using an optoelectronic detector to acquire and fix reflected light scanning, just the reflected light from a point that is optically conjugate with the fundus of the eye being examined can be detected via a confocal aperture such as a pinhole, enabling the effect of unrequired light scattered by the optical system of the eye to be totally excluded.
The confocal optical system has also been used in attempts to detect three-dimensional sectional shapes of the fundus and anterior chamber by two-dimensional laser beam scanning (X and Y scanning) combined with scanning in the direction of the optical axis (Z scanning) (sec Reference (5): SPIE Proceedings Vol. 1028 (1988) pp 127 to 132).
It has been confirmed that a confocal optical system that uses a slit instead of a pinhole is also a highly effective way of improving the quality of fundus images (see Reference (6): JP-A-64-58237, U.S. Pat. No. 4854692, Measurement Science and Technology Vol. 2 (1991) pp 287 to 292).
More recently still, an apparatus has been developed which represents a major advancement, in that it can provide completely real-time detection and display of uneven configurations in the fundus (see Reference (7): JP-A-1-113605, U.S. Pat. No. 4,900,144, Optics Communications Vol. 87 (1992) pp 9 to 14).
Each of these new ophthalmological devices are highly practical because they enable the fundus image to be observed without using a mydriatic to dilate the pupil, despite the relatively small diameter of the pupil. With these devices, the crystalline lens, the anterior chamber and other such regions can be observed by shifting the focal point of the laser beam from the focal plane, the amount of fluorescent agent that needs to be administered when carrying out fluorography of the eye fundus can be considerably decreased, visual function can be examined during observation of the fundus by modulating the scanning laser beam, and a wide range of precise, microscopic examinations of the fundus are possible, using the monochromatic properties of lasers. SLOs are bringing about major innovations in ophthalmology.
However, a major drawback with such devices is the difficulty of the laser beam deflection control system. In References (1) and (3), for example, two mechanical deflectors (swinging mirrors) arc used to scan the laser beam at a horizontal frequency of some 8 kHz and a vertical frequency of 60 Hz (or 50 Hz). Reference (3) also uses a modified system configuration in which the 8 kHz oscillation rate of the horizontal mirror is doubled to enable tracking at a standard TV horizontal scanning frequency of about 16 kHz.
The rapid wear of the mirror suspension bearings caused by this high mirror oscillation frequency of 8 kHz used in those systems to effect the horizontal scanning has an adverse affect on system durability. Over time, shaft wear and fatigue can result in shaft run-out, deviation, hysteresis and other such variations, which, in the case of a SLO in which image quality depends on beam scanning precision, degrades the reliability of the apparatus itself. Another problem with a mirror oscillating at a scanning frequency above about 8 kHz is that in order to achieve the increased oscillation frequency the mirror has to be no more than 5 mm in diameter, and the deflection angle must not exceed 10 degrees or so, which in the case of an eye fundus image system make it impossible to provide high resolution with a wide field of view.
References (2) and (4) use a mirror galvanometer for the low frequency vertical scanning and a rotating polygonal mirror as the deflector for the horizontal scanning. Systems using a polygonal mirror have good beam scanning high-speed characteristics and linearity, and as they are capable of a deflection angle of 20 degrees or more they are better able to provide high image quality with a wide viewing angle than systems that use high frequency vibration mirrors. For full synchronization with the standard NTSC television scanning system, Reference (4) uses a horizontal scanning frequency of 15.75 kHz and a vertical scanning frequency of 60 Hz. In view of current state of the technology, these scanning frequencies are an eminently good choice, and are also practical with reference to interfacing with peripheral equipment.
However, a problem with achieving a scanning frequency of 15.75 kHz is that of the high speed at which the polygonal mirror has to spin, 37,800 rpm in the case of a mirror with 25 facets. In other words, there is still the problem of durability that arises in the case of a mechanical deflector operating at high frequencies, with parts affected by wear and metal fatigue degrading the precision and shortening the service life of the system. With a polygonal mirror, also, image quality can be degraded by unevenness in the laser beam raster caused by shaft play and slight differences in facet trueness and facet division tolerance, and the mirror is also prone to external vibration. A system that uses high-speed rotation needs large bearings and rotation is restricted to a predetermined direction, which make it difficult to reduce the size of the system. A further drawback is the small size that each facet of the mirror is restricted to, so that the scanning is accompanied by an optical shift of the pupil which, in a confocal optical system, results in a reduction of detection efficiency at the end of scan lines and shading of the images.
To avoid the problems relating to durability, shaft run-out and the like that are inherent to mechanical deflectors, for the horizontal scanning the systems of References (6) and (7) each use a non-mechanical acousto-optical deflector (AOD) having no moving parts. An AOD ensures a long service life and highly stable and precise scanning, and also makes it easier to reduce system size.
However, with an AOD the size of the crystal aperture is limited, and generally there are also limitations on the polarization direction that transmits the light, which make it difficult to configure a perfectly confocal optical system using simultaneous double scanning of the incident beam and light reflected by the fundus. Reference (6) therefore describes use of a modified SLO optical system in which the confocal aperture is a slit. Compared to a non-confocal optical system, one that uses a slit aperture offers a marked improvement in image contrast, and is also advantageous in terms of the design of the system apparatus. Even compared to a pinhole (i.e. round aperture) confocal optical system, a slit does not produce much of a difference when the fundus image is observed using short-wavelength visible light (such as blue, green, yellow). However, when long-wavelength light (such as red and infrared) is used, an image obtained with a SLO that uses a slit aperture is closer to what is obtained with a non-confocal optical arrangement, with the contrast of the retinal vessels in the fundus image slightly lower than that obtained with pinhole aperture.
Thus, a problem of an AOD with a slit aperture confocal optical system, especially when using infrared light, is that to some extent it has limited the contrast in images of retinal vessels. The biggest drawback of an AOD deflector is that when high image resolution is required, the AOD has to be constituted of a special substance such as TeO.sub.2 or PbMoO.sub.4, formed into a flat, uniform optical medium with a large-diameter aperture. This usually requires that an anamorphic lens, which is complex to adjust, be disposed to the front and rear of the AOD along the optical axis, and the crystal itself is far more costly than small-aperture media.
With reference to the cost aspect, a high-precision, high-speed swinging mirror or polygonal mirror with pneumatic bearings is also very costly. Thus, the matter of cost and problems concerning deflector scanning performance and reliability are probably what has hindered the practical use and spread of SLOs.
Recent years have also seen the realization of high definition television (HDTV), the aim of which is to provide improved resolution and picture quality, and the feasibility of HDTV compatible SLO systems is being studied. However, with an HDTV system having a horizontal scanning frequency of 30 kHz or more, the above-described problems of each type of deflector arrangement would be correspondingly magnified if attempts were made to operate them at such a high frequency. For this reason, despite the interest in the potential of a HDTV compatible SLO, specific working principles or methods for a commercially practicable system have not yet emerged.
The object of this invention is to provide a scanning laser ophthalmoscope having optical scanning means which combines stable, high frequency operation with long service life, high scanning precision and low cost, thereby reducing the overall cost of the system apparatus, a compact system configuration, a confocal optical system that enables high contrast images to be obtained whatever the wavelength of the light used, and which is also fully HDTV adaptable.